Thanks, LeandroSecundo, I've had a look around for anything which might legitimately be starting the magnifier but can see nothing. It seems to start at random so probably is not linked to a properly established process. I don't think that it's some shortcut which I've tripped over because it sometimes appears while the computer is unattended and, also, a legitimate appearance of the magnifier would surely provide a means to cancel it. Funnily enough, not only is there no advice of how to cancel it and no little box with an "X" in it but my research into the nooks and crannies of all the information available makes no mention of how to turn the thing off once it has been selected intentionally. Once it has appeared unwantedly, I have even gone to the "accessibility" section and selected it "on" in the hope that this action will then make appear a means to turn it off - no such luck! As I said above, I can click on "Exit" but it only closes the magnified version of the entire page which is behind the unmagnified original. It doesn't have any effect upon the small magnified window which follows the cursor around.
I ran the WGA diagnostic (thank you) which showed that everything is completely legal on the computer. I still didn't let it 'phone home to say so, though. Should I or should I just turn WGA off using one of the methods given to me above by fellow conspiracy theorists?
Life used to be so simple....